The subject matter disclosed herein relates generally to radio frequency (RF) coils, and more particularly to a transformer assembly used in an RF coil.
Magnetic Resonance Imaging (MRI) systems use RF coils to acquire image information of a region of interest of a scanned object. The resultant image that is generated shows the structure and function of the region of interest. At least one conventional MRI imaging system includes a multiple-channel array coil having a plurality of coil elements. The signals detected by the multiple-channel array coil are processed by a computer to generate MR images of the object being imaged. During operation, the plurality of coil elements are inductively or capacitively decoupled from the other coil elements. In the majority of the cases the inductive decoupling through overlapping is preferred. When overlapping between elements is not possible, the remote overlapping is performed. Because the setup resembles the classical transformer, the technique of remote inductive decoupling is also called the transformer decoupling. Accordingly, the conventional coil elements are decoupled from one another along a first direction using a transformer decoupling technique. Moreover, the coil elements are decoupled from one another along a second direction using a preamplifier decoupling technique.
The transformer decoupling technique utilizes a conventional transformer that is disposed between each pair of coil elements. The conventional transformer includes a pair of inductor coils that are wound around a cylindrical dielectric. In operation, the mutual inductance of the transformer inductors may have both a positive and a negative effect on the coil elements based on the reciprocal current directions in the coil elements. For example, when the coil element fluxes add up, or have same direction, then the mutual inductance is positive. However, when the current directions are opposed to each other, then the mutual inductance is negative. A positive mutual inductance is typically desired for under-lapped coil elements and a negative inductance is typically desired for overlapped coil elements.
However, inserting the conventional transformer between a pair of coil elements results in an additional inductance being added to the inductance of the coil elements. The additional inductance requires the coil elements to be retuned. Additionally, coupling adjustment of the conventional transformer is difficult. More specifically, the transformer inductors are stretched or compressed to achieve the desired inductance. After the inductor has been formed into a final state, the inductor is coated with a substance to maintain the inductor in the final state. Thus, the conventional transformers are not easily modified to alter the decoupling inductance.